This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. There is a need in high-field/high-frequency-ESR (HFHF-ESR) for resonant structures that maximize the available microwave magnetic field, B1 at the sample since the available power from most millimeter wave sources, is very limited. Recent developments in source technology, however, have allowed us to increase the available power at the resonator by a factor of 4 at 170GHz, corresponding to a doubling of the available B1 at that frequency, and a factor of 10 at 240GHz, corresponding to a trebling of the available B1 at that frequency. Given that the data acquisition time is inversely proportional to the square of available B1, this is extremely significant for samples with finite lifetimes, such as spin-labeled live cells. For biological samples there is also a need for resonant structures and sample holders that minimize the dielectric losses inherent in aqueous samples and exploit suitable geometries for orientation dependent studies of macroscopically aligned membrane samples. We have developed an array of solutions for these problems in applications at 95, 170 and 250GHz. Recent work has focused on developing resonators with variable coupling for optimum sensitivity, and good temperature control for quantitative temperature scans. Progress along these lines has been helped by the development of in-house fabrication techniques for the partially reflecting inductive meshes that we use in semi-confocal Fabry-P[unreadable]rot (FP) resonators at HFHF-ESR frequencies. In particular, the development of a coupling mesh with continuously variable reflectivity has allowed us to critically couple the resonator to a variety of samples, making the optimum use of the available millimeter wave power. Although development work in these areas has been focused primarily on our 95GHz spectrometer, the technology is applicable to all of our HFHF-ESR spectrometers. We have also incorporated those elements of our aqueous sample holder technology facilitating sample positioning within the FP resonator developed at 250GHz into the 95GHz FP resonators we use for achieving the optimum S/N. We have also recently developed flexible probe-head designs that allow our FP resonators to be operated in the induction or reflection mode, depending on the needs of the experiment.